As semiconductor geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology, operate by illuminating a sample with an incident field (typically referred to as a probe beam) and then detecting and analyzing the reflected energy. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in intensity are analyzed.
As shown in FIG. 1, a typical reflectometer includes an illumination source that creates a monochromatic or polychromatic probe beam. The probe beam is projected by one or more lenses onto the surface of a sample. The sample reflects the probe beam and the reflected probe beam (or a portion of the reflected probe beam) is transported to a detector. The detector transforms the energy it receives into corresponding output signals. A processor analyzes the signals to measure the structure or composition of the sample. Reflectometers are often used to measure film thickness and dimensions of etched lines among other properties on semiconductor wafers. Instruments such as these that are designed to measure small spots on specular samples at near-normal incidence will usually use a microscope objective both to focus illumination onto the sample and to collect the reflected light.
An issue with this arrangement is that it can cause the measurements to be sensitive to small errors in local sample tilt. This effect is shown in FIGS. 2 and 3. FIG. 2 shows the typical shape of the pupil of a reflective Schwarzschild microscope objective often used in reflectometers. A cross-section of a portion of a reflectometer containing the objective is shown in FIG. 3. Illumination light 302 reflects from beamsplitter 304 and illuminates the entire objective pupil shown schematically as 306. The light that passes through the pupil is shown as the shaded area 308. This light is focused by the objective shown schematically as 310. The light reflects off a small area of the sample 312 and returns through the objective 310 and pupil 306. Light that passes through the left side of the pupil 306 heading towards the sample 312 will return through the right side and vice versa. The returning light arrives at the beamsplitter 304 where a portion is transmitted. It then continues to a detector (not shown) where at least a portion of the light is collected and measured. In the case of the typical symmetric pupil shown in FIG. 3, the light returning from the sample 312 has the same width as the opening in the pupil through which is must pass. A problem with this typical arrangement is that small variation in sample tilt will cause an undesired variation in the amount of light collected by the detector and therefore degrade the reproducibility of the measurements.
One effective solution to minimize this sensitivity to sample tilt is to place an aperture in the illumination path before the beamsplitter that has openings smaller than those in the objective pupil. A lens may be used to project this aperture onto the back focal plane of the objective to insure good telecentricity. The main drawback with this approach is that it becomes complicated to accommodate multiple objectives, mounted in a turret for example, that may be used in the instrument. A different aperture may be needed for each one. A fixed aperture in the illumination path may also interfere with other configurations of the instrument. It would be very desirable to have a solution for the tilt sensitivity that did not affect other objectives or instrument configurations.
Another problem that is peculiar to the type of pupil shown in FIG. 2 is that the straight “spider arms” that join the central obstruction to the outside diffract light in the vertical and horizontal directions and make it more difficult to measure very small areas on the wafer.
To minimize this type of tilt sensitivity, U.S. Pat. No. 5,486,701 and U.S. Pat. No. 5,747,813 discloses the use of a fully-reflecting mirror covering half the objective pupil together with an aperture to create a beam of illumination that enters only one half of the pupil and exits only from the other half where it can be either slightly larger or smaller than the exit opening. This patent does not disclose how the tilt sensitivity might be remedied when a conventional partially reflecting beam splitter is used to direct light down both halves and collect light from both halves.
For at least these reasons, a need exists for a method for reducing the sensitivity of Schwarzschild (and other normal incidence) objectives to sample tilt. Methods of this type are particularly relevant for metrology systems that use multiple objectives mounted on a single turret. Preferably, such methods would be retrofittable to existing systems and work in combination with traditional beam splitters (i.e., beam splitter that cover the entire objective pupil).